One holding tank which is known for the abovementioned purpose is Australian Patent Specification 595542 and this refers to a holding tank having a layer of particulate material or shellgrit to function as a biological filter which is supported on a perforated tray. The shellgrit may also carry a culture of bacteria which may denitrify contaminants, e.g. faeces produced by aquatic animals in the holding tank. There is also provided water conditioning means associated with the holding tank which includes a pump, a refrigeration unit and water spray unit. The pump may draw water from the tank through the shellgrit and into a conduit which passes through the refrigeration unit and subsequently the spray unit which returns the water to the tank. The water spray may break up any floating scum material into small pieces which may be removed by a skimmer associated wit the tank. The skimmer is connected to the water conditioning means by appropriate valving.
The holding tank arrangement shown in Patent Specification 595542 is efficient in operation and relatively economic to operate. However, the holding tank system as described in Patent 595542 was a fixed or permanent installation and this meant that it was necessary to build a concrete slab so as to support the holding tank with the filter tank being supported on the ground and the components of the water conditioning apparatus also being supported on the ground adjacent the concrete slab. It was a time consuming process to fully install the holding tank system of Patent 595542 since it was not only necessary to form the concrete slab but also set up the water conditioning means including interconnecting the components of the water conditioning means to each other as well as to the filter tank and the holding tank.
The conventional holding tank system as described above especially if a large holding tank was required often needed to be completely installed in concrete and this provided disadvantages to the owner of the holding tank if it was necessary that he transferred his operation to another site. This was frequently necessary if the owner of the holding tank rented or leased the factory which incorporates the holding tank. In such circumstances the demolition which was required to remove the tank could sometimes exceed a week or more and this involved a substantial loss to the owner owing to lack of storage space for the aquatic animals. It will also be borne in mind that it is frequently the case that when a business is expanding or in the reverse situation when a business is contracting that frequent changes of premises may be necessary and this will means that the abovementioned problems in regard to fixed holding tanks for aquatic animals may be exacerbated.